New solutions and comparison of metropolitan area networks
First, the requirements for the next-generation metropolitan area network are given, and then three new schemes of the metropolitan area network are elaborated: Gbit Ethernet, label-based optical burst switching (LOBS) and flexible packet ring (RPR); and analysis of each The advantages and disadvantages of this technology.
Keywords: Ethernet, optical packet switching, LOBS, RPR.
Abstract ： First, we give the basic requirements for the next generaTIon Metropolitan Area Network (MAN). Then, emphases will be put on characterisTIcs and applicaTIons of major three soluTIons such as Gbit Ethernet, Labeled Optical Burst Switching (LOBS) and Resilience Packet Ring (RPR). At last, we evaluate their performances.
Key words: MAN, Ethernet, LOBS, RPR.
1. Overview
At present, the main problem of the metropolitan area network is the bandwidth bottleneck. On the user side, due to the emergence and development of low-cost Gigabit Ethernet, the rate of the local area network has reached a large level. On the long-distance network side, due to the development of DWDM technology, the capacity of commercial systems has reached Tbit / s. The metropolitan area network / access network in the middle becomes the bandwidth bottleneck of the entire network. Secondly, there are multiple overlapping networks in the metropolitan area network. On the one hand, most operating companies currently provide voice and private line services through SDH and circuit switches, and provide data services through SDH and separate frame relay, ATM, and IP networks. Separate network and network technologies often require separate network management systems and Personnel, as well as different configuration and billing systems, result in high equipment costs, high operating costs, and time-consuming and labor-intensive service provision. On the other hand, users must obtain different services through different access technologies and lines, which is not only troublesome but also expensive. Furthermore, most of the current metropolitan area networks use SDH as the transmission platform. Using this fixed bandwidth designed for telephone services to transmit burst data services is not only inefficient, but changing the bandwidth often means changing the physical interface or even changing the service. Types of. In this way, enterprise users often have to redesign and rebuild the network when they need to change their business.
The basic requirements for the next generation metropolitan area network can be summarized as:
Hope to adopt a single public platform to support multi-protocol and multi-service, with the least middle layer;
The current transition from the circuit-switched network to the packet network should be effectively supported, and IP transmission should be optimal in the future;
It is hoped that the network's link capacity and number of nodes can be expanded without restrictions;
With optical transparency, adapt to various existing and future agreements and services;
With topological flexibility, can quickly expand business;
Can realize fast business assignment;
· Integrated, standard and easy-to-use network management system;
Support real-time business represented by traditional voice business;
· Low price (including initial construction and maintenance).
2. Gbit Ethernet
In fact, Ethernet is not a pure new technology, but an "old new technology", mainly used in enterprise networks. The main reason for using Ethernet as a means of access for enterprises and institutions is that there is a huge network foundation and long-term experience and knowledge. All current operating systems and applications are also compatible with Ethernet, with good performance and price ratio, and initial cost. And operating costs are low, good scalability, easy installation and high reliability. Ethernet access adopts asynchronous working mode, which is very suitable for processing IP burst data streams. The technology has important changes and breakthroughs (LAN switching, star wiring, large-capacity MAC address storage, etc.). Its appearance has been greatly improved. Especially after switching from shared media to hub or star structure and using LAN exchange, the information isolation between computers is achieved to a considerable extent. What's more important is to shift Ethernet to full-duplex transmission from then on, eliminating the competition of link bandwidth and potential collision opportunities. Due to the use of dedicated collision-free full-duplex fiber connection, the transmission distance of Ethernet can also be greatly expanded. The most important trend of Ethernet recently is the expansion to metropolitan area networks and even wide area networks. From a technical point of view, Ethernet is a very simple solution that requires only a minimum amount of planning, design, and testing. It has been used for many years and is familiar to users. The business assignment time can be reduced to a few hours or days. Secondly, Ethernet is a standard technology with good interoperability, extensive hardware and software support, and low cost. Some people predict that in metropolitan area networks and wide area networks, the cost of Ethernet may be only 25% and 10% of SDH and ATM. Finally, Ethernet is a media-independent carrier technology that can transparently interface with different transmission media such as copper wire pairs, cables, and various optical fibers, avoiding the cost of rewiring. From a structural point of view, Ethernet is appearing as an unprecedented end-to-end solution, eliminating the format transformation at the network boundary, which is essential for other solutions, and reducing the complexity of the network. Secondly, Ethernet is a very scalable solution, its rate can be extended from 10Mbit / s, 100Mbit / s, 1Gbit / s to 10Gbit / s. From a management point of view, since the same system can be applied at all levels of the network, network management can be greatly simplified. In addition, since many users are already familiar with Ethernet, training is simplified and new services can be expanded faster. Recently, the emergence and development of 10Gbit / s Ethernet has enabled Ethernet technology to substantially increase the capacity while basically maintaining the traditional Ethernet frame structure. It can also interoperate with the existing SDH network at a rate of 10Gbit / s, and for the first time in history has provided a unified and open standard platform that spans LANs, MANs, and WANs.
However, it turns out that Ethernet is used for local area networks, and Qos is not a problem. When trying to expand the application to public telecommunications networks, it is necessary to provide QOS that varies with users. At present, Ethernet has no mechanism to ensure end-to-end performance and cannot provide The QoS required for real-time services and the multi-user sharing node and network necessary billing statistics capabilities. Secondly, Ethernet was originally designed for internal applications of LAN enterprises and users, and lacks the guarantee of security mechanism. Even if there is a demand, it is handled by a high-level agreement. When it is extended to MAN and WAN, the above processing methods using high-level protocols are unacceptable, and new security mechanisms need to be developed. Third, Ethernet is mainly used in a small local area network environment, the network management capability is very weak, and currently there is only a network element-level management system. In the public telecommunications network, it is necessary to effectively operate and maintain a large-scale geographically dispersed network, which requires strong OAM & P capabilities and network-level management capabilities and vision. Fourth, the optical ports of Ethernet switches are directly connected in a point-to-point manner, eliminating the need for transmission equipment, failing to provide fault location and performance monitoring, not supporting the loopback test method, and difficult to implement protection functions. The number of nodes increases rapidly. Finally, although Ethernet is a tried and tested technology as a local area network, it is still an untested new technology for public telecommunications networks, especially WAN environments, and whether its equipment can provide large-scale carrier-grade public networks. The reliability of hardware and software also needs to be verified by practice and time. Generally speaking, traditional Ethernet can be successfully applied to large-scale public telecommunication network environment only after properly solving the above main problems.
3. Label-based optical burst switching (LOBS)
Optical networks based on wavelength routing still have not got rid of the fetters of circuit switching, and the granularity of switching is too coarse (generally at the wavelength level). If it is used to carry an explosively growing data service represented by packets, it lacks flexibility and the utilization rate of optical bandwidth is extremely low. On the contrary, although the packet switching technology shows unique advantages in terms of flexibility and bandwidth utilization, and can share all available bandwidth resources on demand with very fine switching granularity, it is still faced with optical packet + packet switching The cost and some insurmountable technical obstacles, such as packet synchronization technology, packet + packet conflict (competition for resources), and a reasonable and efficient switching structure and packet format, can not enter the practical stage for a while. In the past, the research on packet switching technology mainly focused on the packet + packet switching technology with a fixed length. However, in view of the fact that the current optical signal processing technology is not mature enough, it is impossible to realize the actual situation of all-optical packet switching. The problem of electronic bottlenecks and the improvement of bandwidth utilization are aimed at transmission and switching in the optical domain, and routing and forwarding to process low-rate control information in the electrical domain. At this time, a label-based optical burst switching technology emerged, which combines the advantages of optical "circuit switching" and optical packet switching, while effectively overcoming and avoiding the deficiencies of the two.
Label-based optical burst switching technology is still an IP via MPLS over WDM technology. The basic data exchange / transmission unit in a network using optical burst switching technology is a burst data stream composed of a large number of packets. There are two types of optical packet data flows in this network structure: control packets containing routing information and data packets carrying services. The control information in the control packet must be processed electronically by the network node, while the data packet does not require photoelectric / electro-optical conversion and electronic forwarding of the intermediate node, and is directly transmitted and exchanged in the end-to-end transparent transmission channel. Control packets are transmitted in a specific channel in the WDM transmission link. Each burst of data packets corresponds to a control packet, and the control packets are transmitted before the data packets. The nodes are routed through "datagrams" or "virtual circuits." Mode to assign idle optical channels to burst data streams to achieve dynamic allocation of bandwidth resources of data channels. The control packet transmitted in the first step reserves the necessary network resources for the burst data stream to be transmitted at the intermediate node, and does not wait for the confirmation information of the destination node (just like the establishment process of the virtual circuit in EFA) The burst data stream is sent immediately.
This method of isolating the control packet data channel from the control channel simplifies the processing of burst data exchange, and the control packet length is very short, thus enabling high-speed processing. Optical burst switching technology requires little processing and much lower synchronization overhead processing level than pure packet switching (because it is switched packet by packet, so synchronization is required), it can make full use of the network bandwidth Resources, so in a burst-switched network, the processing load of nodes has been greatly reduced compared to packet switching.
Compared with packet switching and circuit switching technology, burst switching has the following advantages: the granularity of switching is greater than that of packet switching and smaller than that of circuit switching; after the bandwidth is established, there is no need for confirmation at the destination end, and the delay is small; Packet switching must perform a store #forward operation on each intermediate node.
In addition, optical burst switching can set the offset time between the burst data stream and the control packet (that is, the waiting time of the data stream), that is, during the transmission process, the burst can be electronically processed according to the actual status of the link. The data flow is adjusted relative to the delay of the control packet, so neither the control packet nor the data flow needs to perform optical synchronization and optical storage, and can perform QoS functions. It can be seen that this burst switching technology fully utilizes the advantages of the existing photonic technology and electronic technology, the realization cost is relatively low, and it is very suitable for application in the local area network carrying future high burst data services; The capacity of optical burst data flow switching technology can also be used to build backbone networks and metropolitan area networks.
The basic idea of ​​label-based optical burst switching technology and multi-protocol wavelength switching technology in absorbing MPLS label switching technology is the same. The main difference is that label-based optical burst switching separates the control channel from the data switching channel. The information is in the control packet; and the data carried on the wavelength channel at this time is a burst data stream composed of multiple IP packets instead of a single data packet. In a label-based optical burst switching network, each control packet / packet is composed of control information and a label, and is a common IP packet on a label-based optical burst switching path pre-established by running LDP Being transmitted, the path is similar to the label switching path. A plurality of IP packets are assembled into a burst data stream at the label-based optical burst switching entry node, and then the burst data stream is transmitted on the corresponding wavelength channel established by the node after processing the label in the control packet. In the entire forwarding process of the data stream, no electronic operation is required and it is completely performed in the optical domain. Other label operations are similar to the implementation of multi-protocol wavelength switching.
In label-based optical burst switching, each burst data stream corresponds to a label, and each switching node performs electrical processing operations on control information such as label information, wavelength number, and offset time, so different label switching paths The burst data stream can be completely integrated without performing optical / electrical / optical conversion. The technical difficulty of optical burst switching is to find a suitable bandwidth access control protocol, that is, the coordination between control packets and burst data flow, that is, the timing of the removal of a communication connection. At present, the following three methods are mainly used: RFD This method determines the length of time reserved for the current bandwidth by the offset time in the control packet, and immediately disconnects the connection. The advantage is that there is no signaling overhead and it is easy to implement dynamic bandwidth resources The allocation and resource utilization rate are high. An outstanding representative protocol based on this method is the JET protocol; the other is TAC. The protocol is to send control packets to reserve bandwidth first, and then send the burst data stream before Send a packet to release the connection to disconnect the connection; there is another protocol is IBT, which is followed by the IBT logo immediately after the burst data flow, the entire process is reserved bandwidth by the control packet, the IBT logo to remove the connection Therefore, the biggest technical challenge is the all-optical regeneration technology of the IBT logo.
4. Resilient Packet Ring (RPR)
RPR (Resilient Packet Rings) is a standard being developed by the IEEE 802.17 working group to optimize the transmission of data packets on the MAN topology ring. This technology combines the intelligence of IP, the economics of Ethernet and the high bandwidth efficiency and reliability of fiber optic ring networks. The use of space reuse technology, statistical multiplexing and protection ring improves the utilization rate of bandwidth; fully simplifies the network layer, eliminates the repetitiveness of network functions, and minimizes the protocol overhead; it also supports service grading (SLA) and plug-and-play The out-of-the-box features realize the fair use of network resources by nodes.
The network topology is based on two reverse transmission rings. Adjacent nodes are connected by a pair of optical fibers, a pair of wavelengths added and dropped from the WDM link, a SONET / SDH detachable OC-n circuit, and other bidirectional connection media. WDM expansion. Based on this topology, each RPR node supports two ring ports: one supports connection to the neighboring node on the left and the other supports connection to the node on the right. The node only needs to know the optical path status of the two ports. Both the inner and outer rings of RPR are used as working channels to transmit data frames and control frames encapsulated by the RPR protocol. The control frame can be transmitted on the same ring as the data frame, or it can be transmitted on the other ring. But the simulation results show that the latter is more reliable. As can be seen from the network structure, RPR supports unicast, multicast and broadcast transmission, so it is more conducive to the transmission of data services. In addition, when it is found that the node network element or fiber transmission fails, RPR implements a fast automatic protection switching mechanism, and the data will be converted to a fault-free channel within 50ms, which improves the robustness of the network.
The basic structure of RPR is that a buffer is inserted into the ring BIR, and there are three buffers at any node, namely, a sending buffer, a receiving buffer, and a forwarding buffer. If the frame arriving at the node is considered to be local by address matching, the frame is received into the local receive buffer, and if the destination is not local, it is sent out through the forwarding buffer. The frame to be sent by this node sends data through the send buffer. In the case of unicast, RPR supports the space reuse protocol, that is, data frames between different users can be simultaneously transmitted in the ring via different paths on the ring. This is because the transmitted data frames are at the destination node and not like FDDI. In that way, split the network at the source node.
By combining the second layer of simple switching technology with the performance of modern optical network equipment transmission capabilities, bandwidth efficiency, and low protocol overhead, RPR embodies many advantages. Summarized as follows:
1. High bandwidth utilization:
Compared with traditional SDH, RPR greatly improves the efficiency of bandwidth utilization through statistical multiplexing technology, space reuse technology and the use of dark fiber;
2. Fast and robust self-healing ability:
RPR can provide automatic protection switching within 50ms after the fault occurs, providing users with 99.999% of service time. In addition, the priority mechanism of the business flow ensures that the business flow with high priority can be properly processed to meet the needs of real-time business.
3. Providing services for business is simple and fast:
RPR can achieve distributed access to different services on different nodes. Distributed access, fast protection switching and automatic reconstruction of network service functions provide a plug-and-play mechanism for the rapid insertion and deletion of nodes.
In addition, the data communication rate of RPR can reach 1 ~ 10Gbps; the RPR network supports SLA, which can meet the strict requirements of users on the service level and support the end-to-end transmission service level; it fully simplifies the network level and eliminates the duplication of functions; It is easy to manage and operate, and manages resources and traffic in a distributed manner, with rich management information; RPR can also provide new services in a timely manner and quickly upgrade the network.
5. in conclusion
The metropolitan area network should meet the needs of providing users with QoS-guaranteed services quickly, economically, and effectively. Its construction must maintain compatibility with the original network, and must also consider the diversity of service access and the sustainable evolution of the network. This article is not difficult to come up with three new solutions: Gbit Ethernet is simple and economical, but QoS and network management need to be further resolved to be more suitable for WAN. LOBS has high bandwidth and less protocol overhead. However, high cost and immature technology are currently not widely used in WAN. RPR is designed to meet the requirements of the next-generation MAN. Because RPR combines the intelligence of IP, the economy of Ethernet and the high bandwidth efficiency and reliability of optical fiber ring networks, with the further development of standardization work and the further expansion of the market, RPR will surely become a new generation of broadband IP One of the best technologies used by MAN.
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